1. Field of the Invention
The present invention relates to an optical module in the field of optical communication and particularly relates to an optical coupling geometry for bidirectional optical transmission of wavelength-multiplexed signals and a mounting method of such an optical geometry.
Wavelength-multiplexing communication systems are widely applied to communication networks such as a trunk system to deal with increasing network traffic due to rapid growth of the Internet.
In a wavelength-division multiplexing (WDM) system, a plurality of optical signals of different wavelengths are simultaneously transmitted on a single optical fiber. In other words, there are a plurality of channels on a single optical fiber. Therefore, an optical transmission module requires a multiplexing or demultiplexing function (WDM function) for allotting optical signals of different wavelengths and a bidirectional transmission function (send/receive function).
Wavelength multiplexing of optical transmissions is not only applied for trunk communication systems but is also applied for subscriber optical communication systems that extend to office and home environments. FIG. 1 is a diagram showing an example of an optical subscriber communication system that is presently used in practical applications. The system shown in FIG. 1 is a so-called ATM-PON (Passive Optical Network) system with an up-stream transmission of a 1.3 xcexcm band (1260-1360 nm) and a down-stream transmission of a 1.55 xcexcm band (1480-1580 nm). In such a system, transmission is established with one channel for upstream transmission and another channel for downstream transmission.
However, as has been stated above, due to the rapid growth of the Internet, there is a need for providing services at an increased speed and with wider bands in optical subscriber communications.
FIG. 2 shows a system in which the number of wavelengths that are multiplexed is increased to satisfy the need described above. In this system, the 1.55 xcexcm down-stream band is divided to increase number of services to be offered.
On the other hand, a major requirement for optical modules (optical devices) used in such an optical subscriber system is to reduce cost and size.
In order to provide a module that can be applied in the system shown in FIG. 2, a transmitter LD (laser diode), a receiver PD (photo diode), a multiplexing/demultiplexing coupler between the 1.3 xcexcm band and the 1.55 xcexcm band, and a WDM function for dividing the 1.55 xcexcm band are necessary. For the system to become widely used, such functions should be provided with reduced size and cost.
Based on the above, there is an effort of reducing the size and number of components of an optical transmission module and simplifying the assembly process thereof so as to perform mass production at a low cost.
In order to satisfy the above needs, the object of the present invention is to provide a wavelength multiplexing bidirectional optical transmission module of reduced cost and size that can be applied to the optical subscriber communication system of the next generation as shown in FIG. 2.
2. Description of the Related Art
The following description relates to an example of a wavelength multiplexing bidirectional optical transmission module.
FIG. 3 is a diagram showing a structure of a module disclosed in Japanese laid-open patent No. 61-226713 entitled xe2x80x9cOPTICAL WAVELENGTH-TRANSMISSION OPTICAL MODULExe2x80x9d (Example 1 of the related art).
The optical module includes a refractive index distribution type rod lens 235, an optical fiber 212.2 for transmission, which is provided on one end of the rod lens 235, and spacer glasses 216-218 provided on the other end of the rod lens 235, each spacer glass having an interference film filter.
A solid-state light-receiving element (for receiving an optical signal of wavelength xcex3) 224 having a lens 223-1 is provided at a position along an extension of the central axis of the rod lens 235. Further, a solid-state light-emitting element (for emitting an optical signal of wavelength xcex2) 225 having a lens 223-3 and a solid-state light-emitting element (for emitting an optical signal of wavelength xcex1) 226 having a lens 223-2 are provided in radial directions of the rod lens 235.
The interference film filter is made of a short-wavelength pass filter or a long-wavelength pass filter.
With the optical module of the above structure, an optical signal of wavelength xcex3 propagates through the transmission optical fiber 212.2, and is transmitted through the interference film filters 219, 221 and then received at the solid-state light-receiving element 224.
A light beam of wavelength xcex2 from the solid-state light-emitting element 225 is incident on the interference film filter 220 at an angle xcex81. The interference film filter 220 is transparent to a light beam of wavelength xcex2. Then the light beam of wavelength xcex2 is reflected by the interference film filter 219 and is directed to the transmission optical fiber 212.2.
Similarly, a light beam of wavelength xcex1 from the solid-state light-emitting element 226 is incident on the interference film filter 222 at an angle xcex82. Then the light beam of wavelength xcex1 is reflected by the interference film filter 221 and is directed to the transmission optical fiber 212.2.
Accordingly, a three-wave multiplexed bidirectional transmission is achieved.
A more detailed structure of a hybrid-integrated module is known from Japanese laid-open patent application NO. 2000-180671 entitled xe2x80x9cstructure of an optical send/receive module and a fabrication method thereofxe2x80x9d (Example 2 of the related art). FIG. 4 is a diagram showing the structure of such a hybrid integrated module.
An optical fiber 342 is placed inside a ferrule 341. On an end surface of the ferrule 341, a prism-shaped wavelength multiplexing/demultiplexing coupler 343 is fixed that has an interference film filter 344. The interference film filter 344 transmits a light beam of wavelength xcex31 along the optical axis of the optical beam and reflects a light beam of wavelength xcex32 in a direction perpendicular to the optical axis of the light ray.
An LD package having a light-emitting element 322 for emitting a light beam of wavelength xcex3l and a PD package having a light-receiving element 331 for receiving a light beam of wavelength xcex32 are provided along the optical axis and in a direction perpendicular to the optical axis, respectively. Both the LD package and the PD package are fixedly supported by a single housing member 311.
With such a structure, a two-way bidirection transmission is achieved. When the above-described example is applied, a three-wave multiplexing transmission can be achieved using a similar technique.
In the field of optical transmission, there is a need for reducing cost and size of optical transmission devices. However, optical transmission devices include expensive optical modules having an optical multiplexing/demultiplexing function and a photoelectric transfer function. Therefore, there is a requirement for improving functions of optical modules with compact integrated structures and with a simplified assembly process at a low cost.
However, in the above-mentioned related art, there are problems as described below.
The interference film filters used in Examples 1 and 2 of the related art are formed of multiple layers of dielectric materials such as SiO2 and TiO2. In order to achieve a wavelength characteristic in which proximate wavelengths are separated at a high extinction ratio, the number of stacked layers of the dielectric film should be increased while accurately controlling the thickness of each layer.
Therefore, conventionally manufacturing a film for separating proximate wavelengths is feasible but will be extremely expensive.
The number of interference film filters required for Examples 1 and 2 of the related art is greater than or equal to the number of wavelengths to be multiplexed and demultiplexed. Therefore, it is difficult to reduce cost required for multiplexing and demultiplexing wavelength-multiplexed signals.
Also, as can be seen from Example 1 of the related art, a plurality of wavelengths can be coupled and decoupled by dispersing optical paths in a plurality of directions.
Accordingly, since the interference film filters are oriented in a plurality of directions, different types of irregular glass blocks and different types of interference film filters should be manufactured and assembled. This is a drawback from mass production of optical transmission devices.
Further, since a number of directions of optical path dispersion for wavelength multiplexing and demultiplexing is limited, the number of wavelengths that can be multiplexed and demultiplexed is also limited.
Also, for a multiplexing/demultiplexing scheme in which one wavelength is transmitted in one direction, the required number of LD or PD packages described in Example 2 of the related art is equal to the number of directions of wavelength multiplexing (multiplexing/demultiplexing).
Accordingly, the number of directions of wavelength decoupling is also limited due to physical arrangements of the LD or PD packages.
There is another problem in that the LD/PD packages are provided with lead terminals that are directed in a plurality of directions and thus the optical transmission device is not suitable for practical use. Thus, there are various problems in providing a compact optical module.
Accordingly, it is a general object of the present invention to provide an optical transmission module that can obviate the problems described above.
It is another and more specific object of the present invention to provide a wavelength multiplexing bidirectional optical transmission module with reduced size and cost.
In order to achieve the above objects, a wavelength multiplexing bidirectional optical transmission module includes:
a transparent plate having first and second reflection surfaces opposing each other;
a diffraction grating formed on a part of one of the first and second reflection surfaces to receive a wavelength-multiplexed optical signal composed of at least two light beams of a proximate wavelength band and to produce diffracted light beams one for each wavelength at different angles; and
photoelectric transfer elements receiving the diffracted light beams, respectively, that have been reflected and propagated between the first and second reflection surfaces.
According to the above-described structure of the present invention, with an optical module that can be applied for a wavelength multiplexing bidirectional optical transmission, a high functionality is achieved in a hybrid integrated module structure that can be readily manufactured by a conventional fabrication process. Thus, such an optical module can be provided with higher functionality, compact structure and at a reduced cost.